As a motor vehicle steering apparatus, there has been proposed a steer-by-wire system in which mechanical coupling between a steering wheel and a steering mechanism is abolished (Patent Document 1). In the steer-by-wire system, an operation angle of the steering wheel is detected by a sensor, and a steering actuator is controlled corresponding to output of the sensor. The driving force of the steering actuator is transmitted to the steering mechanism. In the steer-by-wire system, it is possible to freely define a ratio of the steering angle of a steerable vehicle wheel relative to the operation angle of the steering wheel(steering angle ratio). Further, steering angle control through control of the steering actuator can be performed independently of the operation of the steering wheel, and this also allows performing of vehicle-behavior stabilizing control by the steering control.
For the motor vehicle steering apparatus capable of changing the steering angle ratio, not only the steer-by-wire system but also, for example, a variable-gear-ratio steering system can be used for the same purpose. The variable-gear-ratio steering system is a steering system in which a variable transmission ratio unit capable of changing a rotation transmission ratio is interposed between a steering wheel and a steering mechanism (Patent Document 2).
In the vehicle-behavior stabilizing control adopted in the steer-by-wire system, etc., feedforward control for defining a steering angle set value (steering-angle target value of a wheel) corresponding to the operation angle of the steering wheel is performed. On the other hand, feedback control is performed in which, based on a vehicle speed and an operation angle, a target yaw rate is calculated, and the steering angle set value is corrected so that a difference between the detected actual yaw rate and the target yaw rate is rendered zero. Thereby, active steering control for turning the wheels irrespective of the operation by a driver is performed, and the stabilization of the vehicle behavior is implemented.
Also, in the steer-by-wire system, etc., a counter-force actuator for imparting an operation counter-force to steering wheels is provided. The counter-force actuator is controlled corresponding to the operation angle of a steering wheel and the vehicle speed. This leads to imparting the steering wheel with the operation counter-force that is similar to the conventional motor vehicle steering apparatus in which the steering wheel and the steering mechanism are mechanically coupled. In this way, a bilateral servo system in which an external force exerted on the steering mechanism side is returned back to the steering wheel is constituted.
The active steering control demonstrates its performance on a slippery road surface such as a snowy road. However, on such a road surface, as compared to a normal road surface, unevenness such as wheel ruts are formed more easily on the road surface. Corresponding thereto, there easily occurs an instance where the vehicle attitude easily becomes unstable, and the vehicle behavior rapidly changes. In such an instance, there may be a case where the vehicle behavior can be guided to a more stable direction if the operation by a driver is reflected, rather than waiting for the active steering control.
Further, on a road surface formed with the aforementioned unevenness, the steering wheel is easily jolted. At this time, it is probable that a counter-force larger than the driver would anticipate is transmitted to the driver via the steering wheel. This may result in a factor that makes the vehicle behavior unstable.    Patent Document 1: U.S. Pat. No. 6,415,215    Patent Document 2: Japanese Unexamined Patent Publication No. 2006-2805